It is generally known in the field of motor vehicle drive technology to use an electric machine as single drive or together with a drive motor of another type (hybrid drive). Polyphase electric machines are typically used as drive motor in such electric or hybrid vehicles. Power electronics which contain an inverter which converts the DC voltage or the direct current from a high-voltage battery on-board the motor vehicle into an alternating current are used in order to actuate such polyphase machines in a motor vehicle. In this case, the inverter is actuated by a control device such that the electric machine, in the motor operating mode, generates a particular torque at a particular rotational speed at an output shaft of the electric machine. Thus, the high-voltage battery of the motor vehicle provides the energy necessary for driving the motor vehicle. For this reason, the high-voltage battery in electric vehicles or plug-in hybrid vehicles must generally be connected via an appropriate battery charging device to an electrical energy supply system, depending on the state of charge, in order to charge the high-voltage battery with electrical energy.
In addition to the battery charging device for charging the high-voltage battery, electric and plug-in hybrid vehicles usually have an on-board power supply system DC-to-DC voltage converter which is designed to feed electrical energy from the high-voltage battery into a low-voltage on-board power supply system (typically with a DC voltage of 12 V). The low-voltage on-board power supply system is used here to supply the consumers found in the motor vehicle and usually has a low-voltage battery which supplies electrical energy to the low-voltage on-board power supply system.
The DC-to-DC voltage converter is operated in a power range of from 1.5 to 2.5 kW, for example. In this case, the maximum current is generally limited on the secondary side (for example to 120 A). The battery charging device for charging the high-voltage battery has a single-phase embodiment as standard and has a typical charging power of approximately 3.5 kW.
A known on-board power supply system topology of vehicle power electronics is designed, for example, such that the charging device and the DC-to-DC voltage converter form independent devices which are connected via the high-voltage battery or an intermediate circuit capacitor. For reasons of safety, both the charging device and the DC-to-DC voltage converter must be designed to be electrically isolated. The electrical isolation is usually realized by means of a transformer. Thus, the charging device can also be implemented in the form of an electrically isolated DC-to-DC voltage converter. In addition to the two DC-to-DC voltage converters, known on-board power supply system topologies have additional voltage adjustment stages (for example boost converters, buck converters) and further electric circuits which are used, for example, to increase the so-called power factor.
If the electric or plug-in hybrid vehicle is connected to the power supply system in order to charge the high-voltage battery, then the low-voltage on-board power supply system must be supported during said charging process since particular consumers (for example pumps, fans, control electronics) are still active. However, the power requirement for supporting the low-voltage on-board power supply system is relatively low and ranges, for example, in a range of from 200 to 400 W. In this case, the efficiency of the energy transfer from the power supply system to the low-voltage on-board power supply system of the motor vehicle is very low since the charging device and the on-board power supply system DC-to-DC voltage converter are connected in series in the on-board power supply system topology described above.
A charging device for a battery-operated vehicle with at least one traction battery for operating an electric motor and at least one on-board battery for supplying an on-board power supply system is known from DE 196 46 666. The described charging device enables integration in terms of circuitry of the charging device and the DC-to-DC voltage converter. A transformer, which has a primary winding and two secondary windings, is used for this purpose. The primary winding of the transformer can optionally be electrically coupled to a supply system or to the traction battery using a changeover device. In a charging operating mode of the charging device, the changeover device is switched such that the traction battery is coupled to the first secondary winding of the transformer and the on-board battery is coupled to the second secondary winding of the transformer.
The translation ratios of the charging device are predefined by the number of turns of the transformer. The primary side of the transformer can be actuated for simultaneous or parallel charging of the traction battery and the on-board battery such that a voltage adjustment takes place for the traction battery. The voltage across the on-board battery then results automatically according to the translation ratio of the transformer. Crucial aspects for the power distribution to the traction battery and the on-board battery are the translation ratios of the transformer, the voltage level of the traction battery and of the on-board battery and the output impedance of the two secondary sides of the transformer. Active control of the power flows from the power supply system to the traction battery or from the power supply system to the on-board battery during parallel charging of the traction battery and the on-board battery is not possible with the known on-board power supply system topologies.
As an alternative to the parallel charging, there exists the possibility of charging the traction battery and the on-board battery sequentially. By way of example, the traction battery can be charged in a first time interval, for example, with a power of 3.5 kW. Then, in a second time interval, the on-board battery can be charged with maximum power (for example with 1.8 kW). However, this leads to heavy cycling of the on-board battery. The discharge-charge cycles taking place shortly one after another cause a reduced service life of the on-board battery. In addition, the sequential charging increases the charging time of the traction battery since the traction battery charging process is repeatedly interrupted for an on-board battery charging process.